What are some threats against which OpenPGP smartcards are useful?

[Christoph Groth]

Hello,

Through an article [1] in LWN, I stumbled across a thread [2] on this
list that dealt with the usefulness of smartcards for storing
OpenPGP keys.

I understand that OpenPGP smartcards do not protect from a compromise
of the computer system that they are used with.  As Peter Lebbing puts
it [3]:

> You don't even have to decrypt the document they're interested in
> yourself, and no external push button will save you. Just decrypt
> a document twice, and the second time, the attacker can use your
> smartcard for their own good while providing the session key they
> logged the first time for your decryption.

But then, what are threats against which smartcards *are* useful?

Robert J. Hansen justifies [4] his use of a smartcard as follows:

> Why don't I want to store the private key on multiple computers?
> Because a good rule of thumb in a forensics lab is "store the minimum
> personal data possible on your systems".

But then he also mentions his 128-bit passphrase and that he would be OK
to publish his (passphrase-protected) private key in a newspaper.  Why
then not store it on the disks of multiple computers?  Because the
decrypted private key could be stolen from RAM by an attacker?  But then
Robert also says that the computer being compromised is a game-over
condition anyway.

I got a smartcard to ssh from computers that I trust reasonably but
where I am not (the only) root to other (more trusted) machines that
I control exclusively and that hold data that I would not store on the
less-trusted machines.  From a fundamental point of view a smartcard
does not provide any additional security here, but I have the
imporession that in practice it does, because gaining access to the
remote machines becomes more difficult for an attacker (without
a smartcard, installing a simple keylogger is enough).  This is the same
kind of imperfect security we rely on in real life, for example with
door locks.  Would you agree with me?

Thanks
Christoph

[1] https://lwn.net/Articles/734767/
[2] https://lists.gnupg.org/pipermail/gnupg-users/2017-April/057995.html
[3] https://lists.gnupg.org/pipermail/gnupg-users/2017-April/058136.html
[4] https://lists.gnupg.org/pipermail/gnupg-users/2017-April/058050.html
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